Torque limiter assembly

ABSTRACT

A torque limiting assembly is disclosed comprising: an input shaft; an output shaft; and spring having a first end coupled to the input shaft and a second end coupled with the output shaft. In a low torque mode, when the input shaft is rotated the spring is compressed by a low amount and transmits a force to drive rotation of the output shaft, whereas in a high torque mode the spring is compressed by a high amount such that the input shaft rotates relative to the output shaft. This relative rotation moves or urges an engagement member so as to either: disconnect the input shaft from the output shaft; or engage a housing to prevent or inhibit rotation of the input shaft relative to the housing.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.17461564.1 filed Jul. 8, 2017, the entire contents of which isincorporated herein by reference.

FIELD

The present disclosure relates generally to assemblies for limiting theamount of torque in a system where an input shaft drives an outputshaft.

BACKGROUND

An overload clutch is a known type of device that mechanically decouplesa rotatable input drive shaft from a rotatable output driven shaft ifthe torque in the system becomes too high due to mechanical overloading.This is used to protect mechanical equipment from damage caused byexcessive torque.

Other types of torque limiting assemblies are also known in which theinput shaft is prevented from rotating if the torque in the systembecomes excessive.

SUMMARY

The present disclosure provides a torque limiting assembly comprising:an input shaft; an output shaft; and a first resilient biasing memberhaving a first portion coupled to the input shaft and a second portioncoupled with the output shaft. The assembly is configured such that, ina low torque mode, when the input shaft is rotated in a first directionthe first resilient biasing member is compressed by a relatively lowamount and transmits a force from rotation of the input shaft to driverotation of the output shaft. The assembly is also configured such that,in a high torque mode, when the input shaft is rotated in the firstdirection the first resilient biasing member is compressed by arelatively high amount such that the input shaft rotates relative to theoutput shaft, thereby moving or urging an engagement member so as toeither: i) disconnect the input shaft from the output shaft; or ii)engage a housing to prevent or inhibit rotation of the input shaftrelative to the housing.

The first resilient biasing member may be selected so as to select thelevel of torque at which the assembly switches between the low torquemode and the high torque mode. For example, in embodiments where theresilient biasing member is a spring, the level of torque at which theassembly switches between the two modes may be set by selecting a springwith a desired spring stiffness.

The first and second portions of the resilient biasing member may be thefirst and second ends of the resilient biasing member, respectively.

The torque assembly comprises said engagement member.

The torque assembly may comprise said housing.

The first portion of the first resilient biasing member may be directlyconnected with the input shaft. The second portion of the firstresilient biasing member may be directly or indirectly coupled/connectedwith the output shaft.

In the low torque mode, the first resilient biasing member is compressedby a relatively low amount such that the input shaft does not rotate, orrotates a relatively low amount, relative to the output shaft. Incontrast, in the high torque mode, the first resilient biasing member iscompressed by a relatively high amount such that the input shaftrotates, or rotates a relatively large amount, relative to the outputshaft.

In use, the input shaft is driven to rotate, e.g. by a motor. It will beappreciated that the low torque mode occurs when there is relatively lowresistance to rotation of the output shaft and the high torque modeoccurs when there is relatively high resistance to rotation of theoutput shaft. Therefore, the first resilient biasing member isconfigured to be compressed a relatively low amount (e.g. not at all)when there is a low resistance to rotation of the output shaft, whereasthe first resilient biasing member is configured to be compressed arelatively high amount when there is a high resistance to rotation ofthe output shaft.

The first resilient biasing member may comprise a spring.

However, it is contemplated that other forms of resilient biasing membermay be used, such as materials that themselves inherently have resilientbiasing properties (e.g. elastic properties), rather than being formedinto a structure that is resiliently biasing.

The first resilient biasing member may be an elongated member thatextends circumferentially around a rotational axis of the input shaft;or the first resilient biasing member may be an elongated member thatextends circumferentially only part way around a rotational axis of theinput shaft.

As described above, the assembly may be configured such that, in a hightorque mode, when the input shaft is rotated in the first direction thefirst resilient biasing member is compressed by a relatively high amountsuch that the input shaft rotates relative to the output shaft, therebymoving an engagement member so as to disconnect the input shaft from theoutput shaft.

The first resilient biasing member enables the input shaft to rotate inthe first direction relative to the output shaft in the high torquemode, i.e. when there is high resistance to rotation of the outputshaft.

The second portion of the resilient biasing member may be coupled withthe output shaft via the engagement member, in the low torque mode.

The first portion of the first resilient biasing member may be connectedto the input shaft, wherein the second portion of the first resilientbiasing member may be in contact with the engagement member, and whereinthe engagement member may be movable between a first position in whichit is connected with the output shaft for rotationally coupling theinput shaft to the output shaft and a second position in which it isdisconnected from the output shaft for rotationally decoupling the inputshaft from the output shaft.

In the low torque mode, the first resilient biasing member may becompressed by said relatively low amount such that it exerts arelatively low force on the engagement member so as to drive rotation ofthe output shaft; and in the high torque mode, the first resilientbiasing member may be compressed by said relatively high amount suchthat it exerts a force on the engagement member that moves it to saidsecond position in which it is disconnected from the output shaft.

The engagement member may be movable between said first and secondpositions in a radial direction with respect to a rotational axis of theinput shaft.

The first position may be radially inwards of the second position.

The first resilient biasing member may comprise a surface in contactwith the engagement member that is at an acute or obtuse angle to theradial direction such that when the first biasing member is urgedcircumferentially in the first direction by the input shaft, ittransmits a force to the engagement member having a component in theradial direction and a component in a circumferential direction.

The angle may be selected so as to determine the circumferential force(i.e. torque) that causes the engagement member to be moved radially todisengage the output shaft.

The output shaft may have a detent in its circumferential surface forreceiving the engagement member when in the first position.

The assembly may comprise a further resilient biasing member arrangedand configured to bias the engagement member into engagement with theoutput shaft in the low torque mode.

The further resilient biasing member may be a spring or other biasingmaterial or member for urging the engagement member radially inwards.

Although the engagement member has been described as being movedradially outwards so as to disengage from the output shaft, it isalternatively contemplated that the engagement member may be movedradially inwards so as to disengage from the output shaft, i.e. suchthat the first position may be radially outwards of the second position.This may be achieved by angling the surface of the first resilientbiasing member differently. In such an embodiment the further resilientbiasing member may bias the engagement member radially outwards intoengagement with the output shaft in the low torque mode.

The end of the input shaft may face an end of the output shaft, i.e. theinput and output shafts may not be concentric.

The assembly may further comprise a second resilient biasing memberhaving a first portion connected to the input shaft and a second portionin contact with the engagement member; wherein the assembly may beconfigured such that, in a low torque mode, when the input shaft isrotated in a second direction the second resilient biasing member iscompressed by a relatively low amount and transmits a force fromrotation of the input shaft to drive rotation of the output shaft; andwherein the assembly is configured such that, in a high torque mode,when the input shaft is rotated in the second direction the secondresilient biasing member is compressed by a relatively high amount suchthat the input shaft rotates relative to the output shaft, therebymoving the engagement member so as to decouple the input shaft from theoutput shaft.

The second resilient biasing member may be arranged and configured tooperate in the same manner as the first resilient biasing member exceptfor when the input shaft is rotated in the second direction instead ofthe first direction.

As described above, the assembly may be configured such that, in a hightorque mode, when the input shaft is rotated in the first direction thefirst resilient biasing member is compressed by a relatively high amountsuch that the input shaft rotates relative to the output shaft, therebymoving an engagement member so as to engage a housing to prevent orinhibit rotation of the input shaft relative to the housing.

The first resilient biasing member enables the input shaft to rotate inthe first direction relative to the output shaft in the high torquemode, i.e. when there is high resistance to rotation of the outputshaft.

The second portion of the resilient biasing member may be fixedlycoupled to the output shaft such that it cannot move relative thereto.

The input shaft may contact the engagement member, wherein the inputshaft may rotates in the first direction relative to the engagementmember in the high torque mode, and wherein the engagement member andinput shaft may be configured such that rotation of the input shaftrelative to the engagement member in the first direction moves or urgesthe engagement member to engage the housing so as to prevent or inhibitrotation of the input shaft relative to the housing.

The input shaft may comprise a portion that is curved or otherwiseramped in a circumferential direction such that rotation of the inputshaft relative to the engagement member in the first direction moves orurges the engagement member radially outwards.

The output shaft may be configured to prevent the engagement membermoving in the first direction relative to the output shaft.

At least a portion of the output shaft may be arranged concentricallywith, and between, at least a portion of the input shaft and at least aportion of the housing; wherein said portion of the output shaft maycomprise an aperture in which the engagement member is arranged, andsaid aperture may be sized and configured to prevent the engagementmember moving in the first direction relative to the output shaft.

The engagement member may be a roller.

The roller may rotate about its own axis in the low torque mode, as theinput and output shafts rotate together relative to the housing. Thisreduces the friction in the system for the low torque mode. In the hightorque mode, the roller is urged or moved against the housing by theinput shaft with sufficient force to stop or inhibit the rotation of theinput shaft. The friction between the roller and the housing in thismode may prevent the roller from rotating about its own axis.

Although only one engagement member for engaging the housing has beendescribed, there may be a plurality of such engagement members andcorresponding components for urging or moving them into contact with thehousing when the input shaft rotates relative to the output shaft in thehigh torque mode.

The assembly may further comprise: a second resilient biasing memberhaving a first portion connected to the input shaft and a second portionfixedly coupled to the output shaft such that it cannot move relativethereto; wherein the assembly may be configured such that, in a lowtorque mode, when the input shaft is rotated in a second direction thesecond resilient biasing member is compressed by a relatively low amountand transmits a force from rotation of the input shaft to drive rotationof the output shaft; and wherein the assembly may be configured suchthat, in a high torque mode, when the input shaft is rotated in thesecond direction the second resilient biasing member is compressed by arelatively high amount such that the input shaft rotates relative to theoutput shaft, thereby moving an engagement member so as to engage ahousing to prevent or inhibit rotation of the input shaft relative tothe housing.

The second resilient biasing member may be arranged and configured tooperate in the same manner as the first resilient biasing member exceptfor when the input shaft is rotated in the second direction instead ofthe first direction.

The present disclosure also provides a torque limiting assemblycomprising: an input shaft; an output shaft; a movable engagement memberbiased to engage the output shaft; and a spring having a first portioncoupled to the input shaft and a second portion coupled with the movableengagement member. The assembly is configured such that, in a low torquemode, rotation of the input shaft transmits a rotational force throughthe spring to the engagement member engaging the output shaft so as todrive rotation of the output shaft. The assembly is also configured suchthat, in a high torque mode, rotation of the input shaft forces thespring to move the engagement member so as to disengage it from theoutput shaft.

The assembly described herein may be used as part of high lift system orother torque transmitting systems. For example, the assembly may be usedin an aircraft. For example, in the embodiments wherein the engagementmember is urged into engagement with a housing, the housing may be anearth ring or a static part of an aircraft structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described, by way of example only, andwith reference to the accompanying drawings in which:

FIG. 1 shows a perspective view of a torque limiting assembly accordingto an embodiment of the present disclosure;

FIG. 2A shows the assembly of FIG. 1 except with the outer housingremoved, and FIG. 2B shows a cross-sectional view of the assembly whenin normal operation;

FIGS. 3A-3C show views of the assembly when the input shaft is beingrotated in a first direction and when the torque in the system isrelatively high, and FIG. 3D shows the assembly when the input andoutput shafts are disengaged;

FIGS. 4A-4B show views of the assembly when the input shaft is beingrotated in a second direction and when the torque in the system isrelatively high, and FIG. 4C shows the assembly when the input andoutput shafts are disengaged;

FIGS. 5A-5B show schematics of another embodiment of a torque limitingassembly; and

FIGS. 6A-6B show schematic perspective views of the assembly, FIGS.6C-6D show cross-sectional views through the assembly, and FIG. 6E showsa sectional view along the length of the assembly.

DETAILED DESCRIPTION

FIG. 1 shows a schematic of a torque limiting assembly according to anembodiment of the present disclosure in the form of an overload clutchassembly. The assembly is configured to disconnect the input shaft 2from the output shaft 3 when the torque becomes excessively high.

The assembly comprises an input drive shaft 2, an output shaft 3 forbeing driven by the input shaft 2, and a clutch mechanism for engagingand disengaging the input shaft 2 and output shaft 3 so as tomechanically connect and disconnect them from each other. The ends ofthe input and output shafts 2,3 that face each other each have flanges5,4. A detent 6 is located in the circumferential surface of the outputshaft flange 4 for receiving an engagement member 8 of the clutchmechanism. The input shaft flange 5 is coupled to the engagement member8 by first and second resilient biasing members 9,10, which in theillustrated embodiment are curved springs. The input shaft flange 5 iscoupled to the engagement member 8 by the curved springs 9,10 such thatin a first mode (low torque mode) the engagement member 8 is in a firstposition and engaged in the detent 6 on the output shaft flange 4,thereby mechanically coupling the input and output shafts 2,3 so thatrotation of the input shaft 2 about its axis drives rotation of theoutput shaft 3 about its axis. The coupling between the input shaftflange 5 and the engagement member 8 by the curved springs 9,10 is alsoconfigured such that in a second mode (high torque mode) the engagementmember 8 is urged to a second position and out of the detent 6 in theoutput shaft flange 4, thereby mechanically disconnecting androtationally decoupling the input and output shafts 2,3 such thatrotation of the input shaft 2 about its axis is not able to drive theoutput shaft 3 to rotate about its axis. These modes are described inmore detail below. The assembly also comprises an inner housing 12 andan outer housing 14 for housing the clutch mechanism.

FIG. 2A shows the assembly of FIG. 1 except with the outer housing 14removed so that the inner housing 12 and clutch mechanism can be seen inmore detail. The inner housing 12 and output shaft 3 are illustrated asbeing translucent purely for illustrative purposes, so that componentsthat would otherwise be hidden by them can be seen more easily.

As described in relation to FIG. 1, the ends of the input and outputshafts 2,3 that face one another each have a radial flange 5,4 and theinner and outer housings 15,14 house the clutch mechanism. As can beseen in FIG. 2A, the engagement member 8 of the clutch mechanism isconnected to a biasing block 16 that is connected to a resilient biasingmember 18, which in the illustrated embodiment is in the form of aspring. The engagement member 8 is biased by the spring 18 so as to urgethe engagement member 8 radially inwards towards the flange 4 of theoutput shaft 3. The engagement member 8 therefore tends to be biasedinto the detent 6 on the output shaft flange 4. Although not shown inFIG. 2A, the outer housing 14 has a pocket on its inner surface in whichthe biasing spring 18 and biasing block 16 are housed, thereby providinga fixed surface from which the spring 18 can urge the biasing block 16and engagement member 8 radially inwards.

As described above, the clutch assembly comprises a first resilientbiasing member in the form of a first curved spring 10 (e.g. an ARCspring) having a first end (first portion) attached to a wedge member 20and a second opposite end (second portion) fixedly attached to theflange 5 of the input shaft 2. The first spring 10 is an elongatedmember that extends circumferentially only part way around a rotationalaxis of the input shaft 1. The wedge member 20 has a tapered surface incontact with a first side of the engagement member. The surface is at anacute or obtuse angle to the radial direction. When the input shaft 2 isrotated in a first direction, the first curved spring 10 resistscompression and transmits the rotational force to the wedge member 20,which in turn transmits the rotational force to the engagement member 8.The bias spring 18 is resistant to compression and, under normaloperating conditions (i.e. in the relatively low torque mode), preventsthe wedge 20 from urging the engagement member 8 radially outwards. Thebias spring 18 also prevents the engagement member 8 from leaving thedetent 6 due to the centrifugal force exerted on it during rotation ofthe assembly. As such, as the input shaft 2 rotates in the firstdirection, the first curved spring 10 and wedge member 20 cause theengagement member 8 to rotate with it. As the engagement member 8 isalso located in the detent 6 on flange 4 of the output shaft 3, theengagement member 8 transmits the rotational force to the output shaft 3such that the output shaft 3 is driven to rotate by the input shaft 2.

As described above, the clutch assembly comprises a second resilientbiasing member in the form of a second curved spring 9 (e.g. an ARCspring). The second spring 9 has a first end (first portion) attached toa second wedge member 22 and a second opposite end (second portion)fixedly attached to the flange 5 of the input shaft 2. The second spring9 is an elongated member that extends circumferentially only part wayaround a rotational axis of the input shaft 1. The wedge member 22 has atapered surface in contact with a second side of the engagement member8, opposite to the first side of the engagement member 8. The surface isat an acute or obtuse angle to the radial direction. When the inputshaft 2 is rotated in a second direction, the second curved spring 9resists compression and transmits the rotational force to the secondwedge member 22, which in turn transmits the rotational force to theengagement member 8. The bias spring 18 is resistant to compression and,under normal operating conditions (i.e. relatively low torque), preventsthe second wedge 22 from urging the engagement member 8 radiallyoutwards. As such, as the input shaft 2 rotates in the second direction,the second curved spring 9 and second wedge member 22 cause theengagement member 8 to rotate with it. As the engagement member 8 isalso located in the detent 6 on flange 4 of the output shaft 3, theengagement member 8 transmits the rotational force to the output shaft 3such that the output shaft 3 is driven to rotate by the input shaft 2.

FIG. 2B shows a cross-sectional view of the assembly when in normaloperation (i.e. under relatively low torque conditions), looking in adirection from the output shaft end towards the input shaft end. It canbe seen that the first and second curved springs 10,9 are housed inrespective curved slots 23,24 within the inner housing 12. In thisnormal (low torque) mode of operation, the biasing spring 18 and biasblock 16 have urged the engagement member 8 radially inwards into thedetent 6 in the flange 4 of the output shaft 3, such that the input andoutput shafts 2,3 are mechanically and rotationally coupled. Asdescribed in relation to FIG. 2A, rotation of the input shaft 2 ineither the first or second direction therefore causes the output shaft 3to rotate in a corresponding direction. In normal operation, the firstand second springs 10,9 have a relatively low level of compression andtransmit the rotational force from the input shaft 2 to the output shaft3 via the engagement member 8.

FIGS. 3A-3C show views of the assembly (with the outer housing 14removed) when the input shaft 2 is being rotated in the first direction(shown by the arrow in FIGS. 3A-3B) and when the torque in the system isrelatively high, i.e. above a threshold value. FIG. 3A shows aperspective view, FIG. 3B shows a cross-sectional view looking from theinput shaft end towards the output shaft end, and FIG. 3C shows across-sectional view of the assembly looking from the output shaft endtowards the input shaft end. As can be seen from FIGS. 3A-3C, when thetorque in the system becomes high, e.g. due to relatively highresistance to rotation of the output shaft 3, the rotational force ofthe input shaft 2 compresses the first spring 10 to a relatively highlevel, allowing the input shaft 2 to rotate angularly relative to theoutput shaft 3. The compression of the first spring 10 increases theforce exerted on the wedge member 20 in a direction towards theengagement member 8, as compared to under the normal operatingconditions described above. The tapered surface of the wedge 20 memberslides under the engagement member 8 and forces the engagement member 8radially outwards, overcoming the force exerted by the bias spring 18.This drives the engagement member 8 out of the detent 6 in the flange ofthe output shaft 3 such that the input shaft 2 is no longer rotationallycoupled to the output shaft 3. As such, rotation of the input shaft 2 inthe first direction no longer drives rotation of the output shaft 3,thus protecting the assembly from being overloaded, which may result indamage. As the input shaft 2 is being rotated in the first direction andnot the second direction, the second curved spring 9 remains relativelyuncompressed as shown best in FIGS. 3B and 3C.

The threshold value of torque which causes the engagement member 8 to bemoved out of the detent 6 is set by the level of compressibility of thefirst spring 10 (and/or optionally by the spring 18). It will beappreciated that levels of torque below the threshold value (caused byrelatively low levels of resistance to rotation of the output shaft)will be absorbed by the first spring 10, with the spring 10 beingcompressed by a relatively low amount that is insufficient to cause thewedge member 20 to displace the engagement member 8 from the detent 6.

FIG. 3D shows the assembly (with the outer housing) when the engagementmember 8 has been disengaged from the detent 6 in the output shaft 3 andthe output shaft 3 has rotated so that the detent 6 and engagementmember 8 are no longer aligned.

FIGS. 4A-4B show views of the assembly (with the outer housing 14removed) when the input shaft 2 is being rotated in the second direction(shown by the arrow in FIG. 4A) and when the torque in the system isrelatively high, i.e. above a threshold value. FIG. 4A shows aperspective view and FIG. 4B shows a cross-sectional view looking fromthe output shaft end towards the input shaft end. As can be seen fromFIGS. 4A-4B, when the torque in the system becomes high, e.g. due torelatively high resistance to rotation of the output shaft 3, therotational force of the input shaft 2 compresses the second spring 9 toa relatively high level, allowing the input shaft 2 to rotate angularlyrelative to the output shaft 3. The compression of the second spring 9increases the force exerted on the second wedge member 22 in a directiontowards the engagement member 8, as compared to under the normaloperating conditions described above. The tapered surface of the secondwedge 22 member slides under the engagement member 8 and forces theengagement member 8 radially outwards, overcoming the force exerted bythe bias spring 18. This drives the engagement member 8 out of thedetent 6 in the flange 4 of the output shaft 3 such that the input shaft2 is no longer mechanically or rotationally coupled to the output shaft3. As such, rotation of the input shaft 2 in the second direction nolonger drives rotation of the output shaft 3, thus protecting theassembly from being overloaded, which may result in damage. As the inputshaft 2 is being rotated in the second direction and not the firstdirection, the first curved spring 10 remains relatively uncompressed asshown best in FIG. 4A.

The threshold value of torque which causes the engagement member 8 to bemoved out of the detent 6 is set by the level of compressibility of thesecond spring 9, optionally and/or the spring 18). It will beappreciated that levels of torque below the threshold value (caused byrelatively low levels of resistance to rotation of the output shaft)will be absorbed by the second spring 9, with the spring 9 beingcompressed by a relatively low amount that is insufficient to cause thewedge member 22 to displace the engagement member 8 from the detent 6.

FIG. 4C shows the assembly when the engagement member 8 has beendisengaged from the detent 6 in the output shaft 3 and the output shaft3 has rotated so that the detent 6 and engagement member 8 are no longeraligned.

FIGS. 5A-5B show schematics of another embodiment of a torque limitingassembly. The assembly is configured to move or urge an engagementmember so as to engage a housing to prevent or inhibit rotation of theinput shaft 30 relative to the housing 32, when the torque becomesexcessively high.

FIG. 5B corresponds to FIG. 5A, except that the outer housing 32 isillustrated as translucent, purely for illustrative purposes, so thatthe internal components may be seen more easily. The assembly comprisesan outer housing 32 in the form of an earth ring, an input (drive) shaft30, an output (driven) shaft 34, a spring carrier 36 and a plurality ofengagement members 38 in the form of rollers, and first and second axialretainers 39,40 at the longitudinal ends of the assembly for retainingthe above-described components therebetween. A portion of the outputshaft 34 is arranged concentrically with, and between, a portion of theinput shaft 30 and a portion of the housing 32.

FIGS. 6A-6B show schematics of the assembly without the earth ring 32and axial retainers 39,40 present, so that the other components can beseen more clearly. In FIG. 6A the spring carrier 36 is shown as beingtranslucent, purely for illustrative purposes, so that other componentsmay be seen more easily. FIG. 6B shows the assembly from another angleand with the output shaft 34 being translucent, purely for illustrativepurposes, so that other components may be seen more easily.

As best seen from FIG. 6A, the input shaft 30 has a radially extendingflange 42 housed within the output shaft 34. As best seen from FIG. 6B,the flange 42 comprises a plurality of curved ramps 44 (detents) spacedcircumferentially around the flange 42 and the output shaft 34 comprisesa corresponding plurality of apertures 46 therethrough that are arrangedat locations radially outward of the curved ramps 44 on the input shaft30. An engagement member 38, which in this embodiment is in the form ofa cylindrical roller member, is located sitting in each of the curvedramps 44 and also within the corresponding aperture 46 in the outputshaft 34.

FIG. 6C shows a cross-sectional view through the assembly at theposition where the rollers 38 are located. It is to be noted that theearth ring 32 is shown in FIG. 6C.

FIG. 6D shows a cross-sectional view through the assembly at theposition where the spring carrier 36 is located. It is to be noted thatthe earth ring 32 is shown in FIG. 6D. This Figure best illustrates thecoupling between the input shaft 30 and spring carrier 36, and thecoupling between the spring carrier 36 and the output shaft 34. Theinput shaft 30 is therefore coupled to the output shaft 34.

It can be seen that the spring carrier 36 is rotationally fixed relativeto the output shaft 34 so that the two rotate together (in normaloperation, i.e. at low torque). For example, the spring carrier 36 mayhave protrusions 48 that interlock with the apertures 46 in the outputshaft 34 (e.g. the ones in which the rollers 38 are located) so as torotationally interlock the spring carrier 36 and output shaft 34. Thespring carrier 36 is coupled to the input shaft 30 so that the inputshaft 30 may rotate to some extent relative to it. A first resilientbiasing member 50, in the form of a first curved spring in theillustrated embodiment, is fixedly attached at one of its ends (at afirst portion) to the spring carrier 36 and is fixedly attached at theother end (second portion) to the input shaft 30, via a first radialprotrusion 52 on the input shaft 30, so as to resist rotation of theinput shaft 30 relative to the spring carrier 36 in a first angulardirection. The first spring 50 extends only partially around thecircumference of the input shaft 30 in a first direction from the firstradial protrusion 52 to the fixation point 53 on the spring carrier 36.A second resilient biasing member 54, in the form of a second curvedspring in the illustrated embodiment, may be provided that is fixedlyattached at one of its ends (at a first portion) to the spring carrier36 and is fixedly attached at the other end (second portion) to theinput shaft 30, via a radial protrusion 55 on the input shaft 30, so asto resist rotation of the input shaft 30 relative to the spring carrier36 in a second angular direction. The second spring 54 extends onlypartially around the circumference of the input shaft 30 in a seconddirection from the radial protrusion 55 to its fixation point 56 on thespring carrier 36. The radial protrusions 52,55 on the input shaft 30may be located in recesses 58 in the interior surface of the springcarrier 36, wherein the recesses 58 and protrusions 52,55 are sized andconfigured so that the input shaft 30 may only rotate relative to thespring carrier 36 until the protrusions 52,55 hit the side walls of therecesses 58 and are prevented from rotating further.

FIG. 6E shows a sectional view along the length of the assembly.

The operation of the assembly will now be described with reference toFIGS. 6A-6E. In operation the input shaft 30 may be rotated about itslongitudinal axis in a first direction. The first spring 50 is resistantto compression and so under normal operating conditions (i.e. underrelatively low torque conditions) the first spring 50 has a relativelylow level of compression. The rotational force from the input shaft 30is therefore transmitted to the spring carrier 36 without significantangular rotation of the input shaft 30 relative to the spring carrier36. The rotational force from the spring carrier 36 is then transmittedto the output shaft 34 so as to rotate the output shaft 36. As there isno significant angular rotation between the input shaft 30 and thespring carrier 36, there is also no significant angular rotation betweenthe input shaft 30 and the output shaft 34. As such, the well of eachcurved ramp 44 on the input shaft 30 remains substantially centrallyaligned with its respective aperture 46 in the output shaft 34 such thatthe respective roller 38 is not urged outwardly by the ramp 44 of theinput shaft 30. This position is best seen in FIGS. 6B and 6C. Therollers 38 are therefore not forced against the earth ring 32 by theinput shaft 30 and so rotation of the input shaft 30 drives rotation ofthe spring carrier 36 and rotation of the output shaft 34.

However, if the torque in the system becomes high and increases to athreshold level, e.g. due to relatively high resistance to rotation ofthe output shaft 34, the rotational force of the input shaft 30compresses the first spring 50 to a relatively high level, allowing theinput shaft 30 to rotate angularly relative to the spring carrier 36 andhence relative to the output shaft 34. As such, the well of each curvedramp 44 on the input shaft 30 rotates relative to its respectiveaperture 46 in the output shaft 34 and relative to its respective roller38, thus forcing the rollers 38 radially outwards and into contact withthe interior surface of the earth ring 32. The rollers 38 may be urgedonto the earth ring 32 by the ramps 44 with such force that the frictionbetween the rollers 38 and the earth ring 32 inhibit or prevent thecircumferential movement of the rollers 38 relative to the earth ring 32and hence inhibit or prevent rotation of the input shaft 30. As such,the torque at the output shaft 34 will be reduced or eliminated.

The threshold value of torque which causes the input shaft 30 to rotaterelative to the output shaft 34 so as to urge the rollers 38 intocontact with the earth ring 32 is set by the level of compressibility ofthe first spring 50. It will be appreciated that levels of torque belowthe threshold value (caused by relatively low levels of resistance torotation of the output shaft 34) will be absorbed by the first spring50, with the spring 50 being compressed by a relatively low amount thatis insufficient to cause the input shaft 30 to rotate relative to theoutput shaft 34 by an amount that causes the rollers 38 to be urged intocontact with the earth ring 32.

Although a frictional engagement has been described, it is alternativelycontemplated that the internal surface of the earth ring 32 may compriserecessed pockets (or other circumferential barriers) configured suchthat when the rollers 38 are forced radially outwards by the ramps 44the rollers 38 engage the pockets (or other barriers) and are unable tomove circumferentially relative to the earth ring 32. This prevents theinput shaft 30 from rotating and hence reduces or eliminates the torqueat the output shaft 34.

The input shaft 30 has been described as being rotated about itslongitudinal axis in a first direction, but it may alternatively berotated about its longitudinal axis in a second direction. The secondspring 54 is resistant to compression and so under normal operatingconditions (i.e. under relatively low torque conditions) the secondspring 50 has a relatively low level of compression. The rotationalforce from the input shaft 30 is therefore transmitted to the springcarrier 36 without significant angular rotation of the input shaft 30relative to the spring carrier 36. The rotational force from the springcarrier 36 is then transmitted to the output shaft 34 so as to rotatethe output shaft 34. As there is no significant angular rotation betweenthe input shaft 30 and the spring carrier 36, there is also nosignificant angular rotation between the input shaft 30 and the outputshaft 34. As such, the well of each curved ramp 44 on the input shaft 30remains substantially centrally aligned with its respective aperture 46in the output shaft 34 such that the respective roller 38 is not urgedoutwardly by the ramp 44 of the input shaft 30. The rollers 38 aretherefore not forced against the earth ring 32 by the input shaft 30 andso the input shaft 30 is free to drive the output shaft 34.

On the other hand, if the torque in the system becomes high andincreases to a threshold level, e.g. due to relatively high resistanceto rotation of the output shaft 34, the rotational force of the inputshaft 30 compresses the second spring 54 to a relatively high level,allowing the input shaft 30 to rotate angularly relative to the springcarrier 36 and hence relative to the output shaft 34. As such, the wellof each curved ramp 44 on the input shaft 30 rotates relative to itsrespective aperture 46 in the output shaft 34, thus forcing the rollers38 radially outwards and into contact with the interior surface of theearth ring 32. As described above, this results in the rotation or theinput shaft 30 being slowed or prevented and a reduction or eliminationof the torque.

The threshold value of torque which causes the input shaft 30 to rotaterelative to the output shaft 34 so as to urge the rollers 38 intocontact with the earth ring 32 is set by the level of compressibility ofthe second spring 54. It will be appreciated that levels of torque belowthe threshold value (caused by relatively low levels of resistance torotation of the output shaft 34) will be absorbed by the second spring54, with the spring 54 being compressed by a relatively low amount thatis insufficient to cause the input shaft 30 to rotate relative to theoutput shaft 34 by an amount that caused the rollers 38 to be urged intocontact with the earth ring 32.

Although the present disclosure has been described with reference tovarious embodiments, it will be understood by those skilled in the artthat various changes in form and detail may be made without departingfrom the scope of the invention as set forth in the accompanying claims.

The invention claimed is:
 1. A torque limiting assembly comprising: aninput shaft; an output shaft; and a first resilient biasing memberhaving a first portion connected to the input shaft and a second portioncoupled to the output shaft, wherein the second portion of the firstresilient biasing member is attached to a wedge member, wherein thewedge member is in contact with an engagement member, and wherein theengagement member is movable between a first position in which it isconnected with the output shaft for rotationally coupling the inputshaft to the output shaft and a second position in which it isdisconnected from the output shaft for rotationally decoupling the inputshaft from the output shaft; wherein the assembly is configured suchthat, in a low torque mode, when the input shaft is rotated in a firstdirection the first resilient biasing member is compressed by arelatively low amount and transmits a force from rotation of the inputshaft to drive rotation of the output shaft; wherein the assembly isconfigured such that, in a high torque mode, when the input shaft isrotated in the first direction the first resilient biasing member iscompressed by a relatively high amount such that the input shaft rotatesrelative to the output shaft, thereby moving or urging the engagementmember so as to disconnect the input shaft from the output shaft; andwherein the assembly comprises a further resilient biasing memberarranged and configured to bias the engagement member into engagementwith the output shaft in the low torque mode.
 2. The assembly of claim1, wherein the first resilient biasing member comprises a spring.
 3. Theassembly of claim 1, wherein the first resilient biasing member is anelongated member that extends circumferentially only part way around arotational axis of the input shaft.
 4. The assembly of claim 1, whereinin the low torque mode, the first resilient biasing member is compressedby said relatively low amount such that it exerts a relatively low forceon the engagement member so as to drive rotation of the output shaft;and wherein in the high torque mode, the first resilient biasing memberis compressed by said relatively high amount such that it exerts a forceon the engagement member that moves it to said second position in whichit is disconnected from the output shaft.
 5. The assembly of claim 1,wherein the engagement member is movable between said first and secondpositions in a radial direction with respect to a rotational axis of theinput shaft.
 6. The assembly of claim 5, wherein the wedge membercomprises a surface in contact with the engagement member that is at anacute or obtuse angle to the radial direction such that when the firstbiasing member is urged circumferentially in the first direction by theinput shaft, it transmits a force to the engagement member having acomponent in the radial direction and a component in a circumferentialdirection.
 7. The assembly of claim 5, wherein the output shaft has adetent in its circumferential surface for receiving the engagementmember when in the first position.
 8. A torque limiting assemblycomprising: an input shaft; an output shaft; and a first resilientbiasing member having a first portion connected to the input shaft and asecond portion coupled to the output shaft, wherein the second portionof the first resilient biasing member is attached to a wedge member,wherein the wedge member is in contact with an engagement member, andwherein the engagement member is movable between a first position inwhich it is connected with the output shaft for rotationally couplingthe input shaft to the output shaft and a second position in which it isdisconnected from the output shaft for rotationally decoupling the inputshaft from the output shaft; wherein the assembly is configured suchthat, in a low torque mode, when the input shaft is rotated in a firstdirection the first resilient biasing member is compressed by arelatively low amount and transmits a force from rotation of the inputshaft to drive rotation of the output shaft; wherein the assembly isconfigured such that, in a high torque mode, when the input shaft isrotated in the first direction the first resilient biasing member iscompressed by a relatively higher amount such that the input shaftrotates relative to the output shaft, thereby moving or urging theengagement member so as to disconnect the input shaft from the outputshaft; and wherein the assembly further comprises: a second resilientbiasing member having a first portion connected to the input shaft and asecond portion is coupled to the engagement member; wherein the assemblyis configured such that, in a low torque mode, when the input shaft isrotated in a second direction the second resilient biasing member iscompressed by a relatively low amount and transmits a force fromrotation of the input shaft to drive rotation of the output shaft; andwherein the assembly is configured such that, in a high torque mode,when the input shaft is rotated in the second direction the secondresilient biasing member is compressed by a relatively higher amountsuch that the input shaft rotates relative to the output shaft, therebymoving the engagement member so as to decouple the input shaft from theoutput shaft.
 9. The assembly of claim 1, wherein the second portion ofthe first resilient biasing member is fixedly coupled to the outputshaft such that it cannot move relative thereto.
 10. A torque limitingassembly comprising: an input shaft; an output shaft; a movableengagement member biased to engage the output shaft; an elongated springhaving a longitudinal axis that circumferentially extends part wayaround a rotational axis of the input shaft, the spring having a firstend coupled to the input shaft and a second end coupled to the movableengagement member; wherein the assembly is configured such that, in alow torque mode, rotation of the input shaft compresses the spring alongits longitudinal axis by a relatively low amounts so as to therebytransmit a rotational force through the spring to the engagement memberengaging the output shaft so as to drive rotation of the output shaft;and wherein the assembly is configured such that, in a high torque mode,rotation of the input shaft compresses the spring along its longitudinalaxis by a relatively higher amount and causes the spring to move theengagement member so as to disengage the engagement member from theoutput shaft.